Both are correct, depending on the context.
The mi is put after the word you're asking about.

Burada bir kız mı var? - would suggest that you were expecting to find a boy here, but it turns out there might be a girl, after all.Burada bir kız var mı? - here, you're simply asking whether or not there is a girl over here.

Senin bir çocuğun mu var? - Do you (really) have a child? (it suggests that, of all the things that you might possess, a child is the last thing I would expect you to have.)Senin bir çocuğun var mı? - Most simply 'Do you have a child?'

Both are correct, depending on the context.
The mi is put after the word you're asking about.

Burada bir kız mı var? - would suggest that you were expecting to find a boy here, but it turns out there might be a girl, after all.Burada bir kız var mı? - here, you're simply asking whether or not there is a girl over here.

Senin bir çocuğun mu var? - Do you (really) have a child? (it suggests that, of all the things that you might possess, a child is the last thing I would expect you to have.)Senin bir çocuğun var mı? - Most simply 'Do you have a child?'

"Burada mı bir kız var?" (Is there a girl here?) Here, the person asking the question is informed that there is a girl around, but he wants to know if this is the place.
"Senin mi çocuğun var?" (You have a child?) Here, the person asking the question can hardly believe the other one is a parent.